1. Field of the Invention
This invention relates to inorganic pigments having improved properties, to their production and to their use for incorporation in lacquers, emulsion paints, plastics, toners, magnetic recording materials, building materials and enamels.
2. Background Information
Inorganic pigments such as, for example, chromium oxides, chromate and molybdate pigments, iron oxides, rutile or spinel mixed phases and ultramarine pigments as well as white pigments, are generally used for coloring paints, plastics and building materials or as pigments for coloring enamels and ceramics. They are generally treated before or during grinding with suitable additions of surface-active substances, which are intended to improve the grinding and pigment properties of the hydrophilic pigments.
Fatty acids and fatty acid esters, alkylamines, vinyl polymers, silicones, etc. are used as surface-active coating materials for those applications of the inorganic pigments where hydrophobic or lipophilic pigment surfaces are required, for example, in pigments for coloring plastics or for use in magnetic recording materials.
Within the group of silicones, polydimethyl or polydiphenyl siloxanes have long been used as coating materials. The disadvantage of these coating materials is that they can gradually become detached from the pigment surface. Accordingly, it has been proposed to use aqueous suspensions of mixtures of silicone oil and water-soluble silicates for hydrophobicizing building materials (W. Noll, Chemie und Technik der Silicone, page 524, Weinheim 1968) or for hydrophobicizing fillers (U.S. Pat. Nos. 2,891,875 and 2,891,923), the silicates being intended to act as adhesion promoters between the silicone oil and the surface of the inorganic materials to be coated.
This silicate-containing coating is unsuitable for inorganic pigments because it adversely affects the processing properties of the pigments in lacquers.
Numerous systems which do not require an expensive silicate/silicone oil double coating have been proposed with a view to obtaining coatings which adhere permanently to the pigment surface.
Thus, EP 0 273 867 describes a composition for improving the hydrophobic properties of inorganic materials which consists of an alkyl trialkoxysilane and an alcohol or hydrocarbon as a solvent and which optionally contains a hydrolysis catalyst. The composition is chemically attached to the surface of the inorganic material to be hydrophobicized and the alkoxysilanes condense to form a polymeric coating.
U.S. Pat. No. 3,849,152 describes the encapsulation of pigments with silicone oils which contain active groups for forming bonds with the pigment surface and which polymerize in situ during the coating process to form a solid coating. These polysiloxanes preferably contain terminal H atoms, OH groups and alkoxy groups and release inter alia alcohols and hydrogen during the coating process.
The polysiloxanes known from EP 257 423, which are likewise used for the coating of pigments and fillers, also form hydrogen. The formation of hydrogen leads to significant technical problems during the production of such coated pigments.
DE 2 543 962 describes a process for improving the dispersion properties of iron oxide magnetic pigments in which, for example, a suspension of iron oxide pigment and ammoniacal ethanol and alkoxysilane is boiled under reflux and dried for several hours at high temperatures.
GB-P 959,211 describes compositions of polyamide fibers containing fillers or inorganic pigments which may be coated with organosilicon compounds. The particular organosilicon compounds with which they have been coated are unimportant to the use of the fillers or inorganic pigments in polyamide fibers. By contrast, the type of surface coating is of considerable importance for inorganic pigments incorporated in lacquers, emulsion paints, plastics, toners, magnetic recording materials, building materials and enamels. Thus, the octamethyl cyclotetrasiloxane used, for example, in GB-P 959,211 is unsuitable because of its poor adhesion properties for the coating of inorganic pigments used in lacquers, emulsion paints, plastics, etc.
Organopolysiloxanes containing alkyl or aryl groups, in which the number of carbon atoms is at most 8 and preferably 1, are described in DE 3 537 415 as hydrophobicizing agents for titanium dioxide used for coloring photographic paper supports. Organopolysiloxanes such as these containing short terminal groups are unsuitable for the coating of inorganic pigments because they do not sufficiently protect the pigment against taking up moisture and pigments thus coated emit dust in large quantities.
All the coatings and coating processes mentioned above are unsatisfactory because they lead to technically unsatisfactory solutions or pollute the environment through the coating process. The coating process should take place in such a way that there is no need for the addition of solvents which have to be subsequently removed.
The chemical attachment of the coating material to the pigment surface with simultaneous elimination of such substances as, for example, alcohols, amines, halogenated hydrocarbons or hydrogen should also be avoided.
In addition, the coated pigments should meet stringent requirements in regard to the stability of their pigment properties and the adhesion of their surface coating.
The pigment processor expects the ground pigments intended for coloring to have favorable coloring properties, adequate fineness from the grinding process and dispersibility and high compatibility with paints, building materials and plastics, irrespective of the nature and duration of storage and irrespective of any temperature and weathering influences during transport and storage.
Accordingly, there is a need for hydrophilic or hydrophobic organic pigments having a stable surface coating which do not lose any of their favorable pigment properties in storage and of which the coating does not decompose or become detached from the pigment surface.
Accordingly, the problem addressed by the present invention was to provide coated pigments which do not have any of the disadvantages mentioned above.